42 · 记录地震
Recording an Earthquake
区分观点、依据、例子与限制，用自己的话准确重建论述。
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An earthquake comes like a thief in the night, without warning. It was necessary, therefore, to invent instruments that neither slumbered nor slept. Some devices were quite simple. One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins. When a shock came, it shook the rigid table upon which these stood. If it were gentle, only the more unstable rods fell. If it were severe, they all fell. Thus the rods, by falling, and by the direction in which they fell, recorded for the slumbering scientist the strength of a shock that was too weak to waken him, and the direction from which it came.
But instruments far more delicate than that were needed if any really serious advance was to be made. The ideal to be aimed at was to devise an instrument that could record with a pen on paper, the movements of the ground or of the table as the quake passed by. While I write my pen moves, but the paper keeps still. With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved. That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments seismometers for recording earthquake waves. But when table, penholder and paper are all moving, how is it possible to write legibly? The key to a solution of that problem lay in an everyday observation. Why does a person standing in a bus or train tend to fall when a sudden start is made? It is because his feet move on, but his head stays still. A simple experiment will help us a little further. Tie a heavy weight at the end of a long piece of string. With the hand held high in the air, hold the string so that the weight nearly touches the ground. Now move the hand to and fro and around but not up and down. It will be found that the weight moves but slightly or not at all. Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper on the floor. Imagine an earthquake shock shaking the floor, the paper, you and your hand. In the midst of all this movement, the weight and the pen would be still. But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface. It was upon this principle that the first instruments were made, but the paper was wrapped round a drum which rotated slowly. As long as all was still, the pen drew a straight line, but while the drum was being shaken, the line that the pen was drawing wriggled from side to side. The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated. If we could actually see the path described by a particle, such as a sand grain in the rock, it would be more like that of a bluebottle buzzing round the room; it would be up and down, to and fro and from side to side. Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
When the instrument is situated at more than 700 miles from the earthquake centre, the graphic record shows three waves arriving one after the other at short intervals. The first records the arrival of longitudinal vibrations. The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first. These two have travelled through the earth. It was from the study of these that so much was learnt about the interior of the earth. The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.
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整篇课文
An earthquake comes like a thief in the night, without warning. It was necessary, therefore, to invent instruments that neither slumbered nor slept. Some devices were quite simple. One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins. When a shock came, it shook the rigid table upon which these stood. If it were gentle, only the more unstable rods fell. If it were severe, they all fell. Thus the rods, by falling, and by the direction in which they fell, recorded for the slumbering scientist the strength of a shock that was too weak to waken him, and the direction from which it came.
But instruments far more delicate than that were needed if any really serious advance was to be made. The ideal to be aimed at was to devise an instrument that could record with a pen on paper, the movements of the ground or of the table as the quake passed by. While I write my pen moves, but the paper keeps still. With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved. That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments seismometers for recording earthquake waves. But when table, penholder and paper are all moving, how is it possible to write legibly? The key to a solution of that problem lay in an everyday observation. Why does a person standing in a bus or train tend to fall when a sudden start is made? It is because his feet move on, but his head stays still. A simple experiment will help us a little further. Tie a heavy weight at the end of a long piece of string. With the hand held high in the air, hold the string so that the weight nearly touches the ground. Now move the hand to and fro and around but not up and down. It will be found that the weight moves but slightly or not at all. Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper on the floor. Imagine an earthquake shock shaking the floor, the paper, you and your hand. In the midst of all this movement, the weight and the pen would be still. But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface. It was upon this principle that the first instruments were made, but the paper was wrapped round a drum which rotated slowly. As long as all was still, the pen drew a straight line, but while the drum was being shaken, the line that the pen was drawing wriggled from side to side. The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated. If we could actually see the path described by a particle, such as a sand grain in the rock, it would be more like that of a bluebottle buzzing round the room; it would be up and down, to and fro and from side to side. Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
When the instrument is situated at more than 700 miles from the earthquake centre, the graphic record shows three waves arriving one after the other at short intervals. The first records the arrival of longitudinal vibrations. The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first. These two have travelled through the earth. It was from the study of these that so much was learnt about the interior of the earth. The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.
地震来得像夜里的窃贼，事先毫无警告。因此，人们需要发明既不打盹、也不睡觉的仪器。有些装置相当简单。例如，有一种装置由长短、粗细不同的杆子组成，像九柱戏的木柱一样竖立着。地震发生时，摆放杆子的硬桌会受到震动。震动轻微，只有较不稳定的杆子倒下；震动剧烈，杆子就全倒了。这样，即使地震轻微得不足以惊醒熟睡的科学家，杆子倒下的情况和方向，也能替他记下震动的强弱及其传来的方向。
不过，要取得真正重要的进展，还需要精密得多的仪器。理想的目标，是设计一种装置，在地震波经过时，能用笔把地面或桌子的运动记录在纸上。我写字时，笔在动，纸不动。经过练习，我大概也能学会让笔不动、纸来动，照样把字写出来。这个想法听起来很傻，却正是一些早期地震仪记录地震波所采用的思路。可是，当桌子、笔架和纸都在动时，怎样才能留下清晰的记录呢？解决问题的关键，就在一个日常可见的现象里：公共汽车或火车突然启动时，站着的人为什么容易跌倒？因为脚随车向前移动，头部却还停留在原来的位置。一个简单的实验，可以帮助我们进一步理解。把一个重物系在长绳的一端，举高手，握住绳子，让重物几乎碰到地面。然后前后、左右或绕圈移动手，但不要上下移动。你会发现，重物只稍稍移动，甚至根本不动。设想在重物上固定一支笔，让笔尖落在地板上的一张纸上。再设想地震使地板、纸、你和你的手都摇晃起来。在这一片运动中，重物和笔却保持不动。纸在笔尖下左右移动，笔墨便把它的运动记录在纸面上。最初的仪器正是按这一原理制造的，只是把纸绕在一个缓缓转动的圆筒上。一切静止时，笔画出的是直线；圆筒受到震动时，笔留下的线条就左右扭动。不过，上述装置只能记录地震波运动的水平分量，实际的波动要复杂得多。如果真能看到一个质点——例如岩石中一粒沙子——的运动轨迹，它会更像一只在屋里嗡嗡乱飞的苍蝇：既上下运动，又前后、左右移动。人们已经设计出相应的仪器，并可通过适当安放，把这三个方向的运动分别记录成不同的曲线。
当仪器位于距地震中心七百多英里以外的地方时，记录图上会显示三种地震波，它们以较短的间隔先后到达。最先记录到的是纵向振动；随后到达的是横向振动，传播速度较慢，比前者晚几分钟。这两种波都是穿过地球内部传来的。正是通过研究它们，人们才了解到许多地球内部的情况。第三种波，也就是主波，传播最慢，是沿着地球表层的岩石绕行而来的。
学完这一课
介绍一种设备分别测出什么，并清楚列出测量维度。
译文依据整理稿，由本站按中文表达重新校订；解析据吴应时整理资料及首轮审校稿；词伙与练习提示为本站学习设计。原PDF第184—190页，2022年2月第2版。